1. Field of the Invention
The present-invention relates to multimetal oxide compositions of the formula I EQU [A].sub.p [B].sub.q (I),
where
A is Mo.sub.12 X.sup.1.sub.a X.sup.2.sub.b X.sup.3.sub.c X.sup.4.sub.d S.sub.e X.sup.5.sub.f O.sub.x co-phase PA1 B is X.sup.6.sub.12 Cu.sub.g X.sup.7.sub.h O.sub.y key-phase PA1 X.sup.1 is phosphorus, arsenic, boron, germanium and/or silicon, PA1 X.sup.2 is vanadium, niobium and/or tungsten, PA1 X.sup.3 is hydrogen, of which up to 97 mol % may have been replaced by potassium, rubidium, cesium and/or ammonium (NH.sub.4), PA1 X.sup.4 is antimony and/or bismuth, PA1 X.sup.5 is rhenium and/or rhodium, PA1 X.sup.6 is molybdenum, tungsten, niobium and/or tantalum, PA1 X.sup.7 is lithium, sodium, potassium, rubidium, cesium and/or ammonium (NH.sub.4), PA1 a is from 1 to 6, preferably from 1 to 3, particularly preferably from 1.5 to 2.5 PA1 b is from 0 to 6, preferably from 0.2 to 4, particularly preferably from 0.5 to 2 PA1 c is from 3 to 5, PA1 d is from 0 to 6, preferably from 0 to 3, particularly preferably from 0.5 to 1.5 PA1 e is from 0 to 3, preferably from 0.01 to 1, particularly preferably from 0.01 to 0.2, PA1 f is from 0 to 3, preferably from 0.01 to 1, particularly preferably from 0.01 to 0.5, PA1 g is from 4 to 24, preferably from 5 to 18, particularly preferably from 8 to 15, PA1 h is from 0 to 20, preferably from 0 to 12, particularly preferably from 6 to 12, PA1 x and y are numbers determined by the valency and frequency of the elements other than oxygen in I, and PA1 p and q are numbers other than zero whose ratio p/q is from 12:0.1 to 12:48, preferably from 12:0.25 to 12:12, particularly preferably from 12:0.5 to 12:4, PA1 (NH.sub.4).sub.2 Cu(MoO.sub.4).sub.2 (Card 40-1490 of the JCPDS-ICCD Index (1991)), PA1 (NH.sub.4).sub.2 Cu(MoO.sub.4).sub.2 (NH.sub.3).sub.2 (Garin, L. & Costamaga J., Powder Diffraction Vol. 4, No. 4 (1989) P. 233), PA1 NaCu(OH)(MoO.sub.4) (Clearfield et al., Inorg. Chem. 25 (1986) P. 3782), PA1 CuMoO.sub.4 (Card 22-242 of the JCPDS-ICDD Index (1991)), PA1 Cu.sub.2 MoO.sub.5 (Card 22-607 of the JCPDS-ICDD Index (1989)), PA1 Cu.sub.2 MO.sub.3 O.sub.10 (Card 35-16 of the JCPDS-ICDD Index (1991)), PA1 Cu.sub.3 Mo.sub.2 O.sub.9 (Card 24-55 and 34-637 of the JCPDS-ICDD Index (1991)), Cu.sub.3 (MoO.sub.4).sub.2 (OH).sub.2 (Lindgrenit, Card 36-405 of the JCPDS-ICDD Index (1991)), PA1 Cu.sub.4 Mo.sub.5 O.sub.17 (Card 39-181 of the JCPDS-ICDD Index (1991)), PA1 Cu.sub.4-x Mo.sub.3 O.sub.12 where x=0 to 0.15 (Cards 24-56, 26-547 and 35-18 of the JCPDS-ICDD Index (1989)), PA1 Cu.sub.4 Mo.sub.6 O.sub.20 (Clearfield et al., Inorg. Chem. Vol. 25 (1986) P. 3782)), PA1 Cu.sub.6 Mo.sub.4 O.sub.15 (Cards 38-1380 and 35-17 of the JCPDS-ICDD Index (1991)), PA1 Cu.sub.6 Mo.sub.5 O.sub.18 (Card 40-865 of the JCPDS-ICDD Index 1991)); PA1 Mo: ammonium heptamolybdate, PA1 V: ammonium metavanadate, PA1 p: from 70 to 100%, preferably from 76 to 85%, phosphoric acid, PA1 Sb: senarmontite, PA1 S: ammonium sulfate, PA1 Re: rhenium pentoxide or ammonium perrhenate, PA1 B: boric acid, PA1 As: arsenic trioxide, PA1 Si: waterglass, PA1 Nb: ammonium niobium oxalate or ammonium niobate, PA1 NH.sub.4 : ammonium sulfate, nitrate or carbonate, PA1 Bi: bismuth nitrate.
which contain component [A].sub.p in the form of three-dimensionally extended regions A of the chemical composition EQU A Mo.sub.12 X.sup.1.sub.a X.sup.2.sub.b X.sup.3.sub.c X.sup.4.sub.d S.sub.e X.sup.5.sub.f O.sub.x (co-phase)
which are delimited from their local environment due to their chemical composition which is different from their local environment, and component [B].sub.q in the form of three-dimensionally extended regions B of the chemical composition EQU B X.sup.6.sub.12 Cu.sub.g X.sup.7.sub.h O.sub.y (key phase)
which are delimited from their local environment due to their chemical composition which is different from their local environment, where the regions A and B are distributed relative to one another as in a mixture of finely divided A and finely divided B.
The present invention also relates to a process for the preparation of these compositions, and to their use.
2. Description of the Prior Art
EP-A 446 644 relates to multimetal oxide compositions whose empirical elemental composition corresponds to that of the novel multimetal oxide compositions.
These multimetal oxide compositions are prepared by converting suitable sources of the constituents of the desired multimetal oxide compositions in the requisite amounts into an intimate dry mix, and subsequently calcining the latter at elevated temperature for several hours. The resultant multimetal oxide compositions are recommended, inter alia, as catalysts for the preparation of methacrylic acid from methacrolein by gas-phase catalytic oxidation. However, the multimetal oxide compositions of this prior art have the disadvantage that both their activity and the selectivity in the formation of methacrylic acid for a given conversion are not entirely satisfactory. The same applies to the reproducibility of their preparation and to their service lives, which are particularly unsatisfactory if the reaction gases comprising methacrolein as the principal constituent contain organic acids as secondary constituents.
EP-A 835, DE-C 3 338 380, DE-A 42 20 859 and the earlier application DE-A 43 07 381 (O.Z. 0050/43890) likewise relate to multimetal oxide compositions which are suitable as catalysts for the preparation of methacrylic acid from methacrolein by gas-phase catalytic oxidation and which advantageously likewise have a key phase/co-phase structure. Although the general formulae of this prior art, amongst a broad diversity of possible multimetal oxide compositions, formally also cover those whose key phase can contain copper in addition to elements such as molybdenum and tungsten and whose co-phase can simultaneously contain, for example, phosphorus, arsenic or antimony, none of the working examples includes a single working example of this type, but instead are restricted to those whose key phase contains bismuth instead of copper. This embodiment is emphatically recommended by the prior art as the particularly preferred one, in particular also as catalyst for the catalytic gas-phase oxidation of methacrolein to methacrylic acid. However, this preferred embodiment of the prior art has the disadvantage that it is not entirely satisfactory as a catalyst for the catalytic gas-phase oxidation of methacrolein to methacrylic acid.